The present invention relates in general to printed monopulse primary sources for antennas. The present invention provides a printed monopulse primary source construction that is especially well-suited for use in airborne radar antennas.
Monopulse primary sources generally comprise a radiating circuit including a plurality of radiating elements fed with electromagnetic energy from a feeding/receiving circuit. The radiating elements are spatially positioned and respectively fed with electromagnetic energy having predetermined phase relationships so as to form a sum channel .SIGMA. and one or two difference channels .DELTA., such as elevation and azimuth difference channels .DELTA.S and .DELTA.G.
Most known monopulse primary sources providing sum and difference channels fall into one of two general types. The first such general type of monopulse source is constructed by associating a plurality of metallic wave guides. Generally, these wave guides have a rectangular cross-section. In one specific case of this first general type, a plurality of over-dimensioned wave guides are associated in which higher order propagation modes are produced. Such construction enables error signals to be generated in the elevation and azimuth planes.
Wave guides sources are difficult to design on paper and even more difficult to fabricate into practical constructions. They are expensive and cumbersome. Their length can be three (3) to ten (10) times the wavelength of the electromagnetic signal to be transmitted or received, depending on the complexity of the source. Wave guide monopulse sources have been described in such publications as "Les Antennes" by L. Thourel published in 1971 by Dunod (see chapter 9) and in "Multi-mode Antennas" by S. W. Drabowich published in the Microwave Journal dated January 1966 (see pp 41-51).
The second general type of known monopulse primary source providing sum and difference channels uses a microstrip printed feeding/receiving circuit. These microstrip printed circuit monopulse sources include microstrip radiating elements. Each radiating element includes a conducting plate positioned over a ground plane and spaced therefrom with a dielectric material. In the prior art, such radiating elements are generally fed by a microstrip circuit including one or more 6.lambda./4 hybrid junctions grouped on the same side of the dielectric material substrate that carries the radiating elements. Known hybrid circuits are described in U.S. Pat. Nos. 3,921,177--Munson (Nov. 18, 1975) and 3,811,128--Munson (May 14, 1974).
Referring to FIG. 1, there is shown a typical prior art microstrip monopulse source. The source includes four (4) radiating elements A, B, C, and D. The four radiating elements are connected in circuit by four (4) hybrid junctions a, b, c, and d. The radiating elements and hybrid junctions are printed on opposite sides of a double-sided metallized dielectric substrate. The radiating elements A, B, C and D are fed by the hybrid junctions through impedance transformers such as impedance transformer 40.
Output 41 of junction a delivers a signal (A-B) to junction d. Output 42 of junction a delivers the signal (A+B) to junction b. Output 43 of junction c delivers a signal (D+C) to junction b and output 44 of junction c delivers a signal (D-C) to junction d. Outputs 45 and 46 of junction b, supplied respectively by signals (A+B) and (D+C), provide a sum signal .SIGMA.=0(A+B)+(C+D) and an elevation difference signal .DELTA.S=(A+B)-(D+C). An output 47 of junction d supplied by signals (D-C) and (A-B) provides the azimuth difference signal .DELTA.G=(A-B)+(D-C)=(A+D)-(B+C). Output 48 of junction d is loaded.
Using this known circuit the signal path of sum channel .SIGMA. crosses two junctions in cascade thereby incurring a substantial signal loss. In addition, it is possible to obtain undesirable couplings among the channels which degrades performance of the printed monopulse source.